Sensitivity of Arctic climate simulations to different boundary-layer parameterizations in a regional climate model

Arctic climate simulations with the high resolution regional climate model HIRHAM show some deviations from station data in the planetary boundary layer ( PBL) during a winter, which indicates the necessity of improvements in the atmospheric PBL parameterization for a better description of the vertical stratification and atmosphere surface energy exchange. A 1-dimensional single column model scheme has been ur;ed to investigate the influence of two different PBL parameterizations in monthly integrations for January 1991 and July 1990. The first scheme uses the boundary layer parameterization of the atmospheric circulation model ECHAM3, including the Monin-Obukhov similarity theory in the surface layer and a mixing length approach above. The second scheme applies the Rossby-number similarity theory fbr the whole PBL, connecting external parameters with turbulent fluxes and with universal Functions determined on the basis of Arctic data. For both schemes the heat and humidity advection has been determined as residual term of the PBL balance equations. Diabatic sources have been computed from the current model solution and local temperature and humidity changes are estimated from radiosonde data. The simulated vertical structure and the atmosphere surface energy exchange during January strongly depends on the used PDL parameterization scheme. These different PBL parameterization schemes were then applied for simulations of the Arctic climate in the 3-dimensional regional atmospheric climate model HIRHAM, using ECHAM3 with Monin-Obukhov similarity theory, ECHAM3 with Rossby-number similarity theory and ECHAM4 parameterizations with a turbulent kinetic energy closure. The near surface temperature, the large-scale fields of geopotential and horizontal wind are simulated satisfactorily by all three schemes, but strong regional differences occur. The results show a sensitivity to the type of turbulence exchange scheme used. The comparison with ECMWF analyses and with radiosonde data reveals: that during January ECHAM3 with Rossby number similarity theory more successfully simulates the cold and stable PBL over land surfaces, whereas over the open ocean ECHAM3 with Monin Obukhov similarity works better. ECHAM3 with Rossby-number similarity theory delivers a better adapted vertical heat exchange under stable Arctic conditions and reduces the cold bias at the surface. The monthly mean surface turbulent heat Bur distribution strongly depends on the use of different PBL parameterizations and lends to different Arctic climate structures throughout the atmosphere with the strongest changes at the ice edge for January.